In conventional lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise a top layer applied over the hydrophilic surface of a substrate. The top layer typically comprises one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions are removed in the developing process revealing the underlying hydrophilic surface of the substrate, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative-working. In each instance, the regions that remain (i.e., the image areas) are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, the dimensions of the mask may change slightly due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging of imageable elements, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Positive working, thermally imageable, multi-layer elements are disclosed, for example, in Shimazu, U.S. Pat. No. 6,294,311, and U.S. Pat. No. 6,352,812; Patel, U.S. Pat. No. 6,352,811; and Savariar-Hauck, U.S. Pat. No. 6,358,669, and U.S. Pat. No. 6,528,228; the disclosures of which are all incorporated herein by reference.
However, in the preparation of multi-layer imageable elements, it is typically necessary to coat a top layer over an underlayer. To prevent the underlayer from dissolving in the coating solvent for the top layer and mixing with the top layer when the top layer is coated over the underlayer, care must be taken in selecting the components used in the top layer, the underlayer, and the coating solvent for the top layer. However, mixing of the layers, which can cause ablation of the top layer during imaging, may occur. In addition, the need to use ingredients in the underlayer that will not dissolve in the coating solvent for the top layer and the need to use coating solvents for the top layer that will not dissolve the underlayer, limit available formulation choices.
Poly(methyl methacrylate) and copolymers of methyl methacrylate are excellent top layers for multilayer imageable elements. Poly(methyl methacrylate) is very unpenetrable to many developers in the unexposed regions. It is typically soluble in solvents in which the underlayer ingredients are insoluble, so that layer intermixing does not occur during preparation of the imageable element. However, poly(methyl methacrylate) from the imaged regions may leave residual “skins” of poly(methyl methacrylate) throughout the development tank of a processor, which may block the pipes and pumps of the processor and/or re-deposit on the non-image regions of the developed printing plate. Thus, a need exists to improve the developability of multilayer imageable elements that contain poly(methyl methacrylate) and/or copolymers of methyl methacrylate in the top layer.